Miniature electrical fuse



Nov. 12, 1963 J. M. BORZONI MINIATURE ELECTRICAL FUSE Filed Dec. 14, 1960 f A 5 I.

United States Patent 3,110,787 MENIATURE ELECTRICAL FUSE .fohn Martin Borzoni, Des Plaines, Ill., assignor to Littelfuse Incorporated, Des Plaines, IlL, a corporation of Illinois Filed Dec. 14, 1960, Ser. No. 75,710. 6 Claims. (Ill. 200-113) This invention relates to electrical fuses. The most important application of the invention is in miniature fuses usable with printed circuits or other electrical apparatus where compactness is a primary consideration, although many aspects of the invention are useful in conventional circuits where miniaturization is not necessarily a requirement.

An example of a miniature fuse is one having a thickness in the order of 0.2 inch or less and a length in the order of 0.3 inch or less. Hereto-fore, the fabrication of miniature fuses to close resistance tolerances using mass production techniques have presented many problems due in part to the construction and arrangement of the parts making up the miniature fuses. Miniature fuses used in circuits where high short circuit current and voltage levels are present have been subject to arcing which builds up exceedingly high temperature and pressure conditions in the small space within the fuse which cause the fuse to explode.

it is, accordingly, an object of the present invention to provide a reliable miniature fuse which may be readily mass produced to close resistance tolerances. A related object of the invention is to provide a fuse, preferably a miniature fuse usable at high current and voltage levels,

and wherein arcing is reduced or held to low energy levels. A related object of the invention is to provide a miniature fuse as described which is ruggedly constructed so as to withstand moderately high pressure and temperature conditions therein without danger of exploding.

Another object of the invention is to provide a simple and efiicient method of making fuses having the advantages described above.

In its most advantageous form, the fuse of the present invention is a miniature fuse having a body made of a molded synthetic plastic insulating material in which is embedded a pair of spaced, juxtaposed, parallel lead wires exposed at opposite ends of the insulating fuse body. The lead wires may project a short distance beyond one end of tie fuse body, where they have sufiicient rigidity to constitute plug connectors insertable into a suitable socket connector, or they may extend a relatively large distance ther-ebeyond where they are flexible to form wrap-around connecting leads for solder connections. A fuse filament extends between the other exposed ends of the lead wires. A cup-shaped cap member encloses the fuse filament carrying end of the fuse body to seal off a space containing the fuse filament. To enable the fuse to be used at high voltage and current levels, the insulating material of the fuse body in which the lead wires are embedded is extended around the lead wires to a point adjacent and, in one embodiment of the invention, to the very end of the lead wires so that a minimum of lead wire surface is exposed to the space above the fuse body. The fuse body insulation material around the ends of the lead wires most advantageously form spaced insulating posts.

Arcing problems occur, of course, when the fuse filament initially disintegrates (i.e. melts and vaporizes) under a high overload. At low power levels, arcing is not a serious problem even in miniature fuses. But, in 125 volt power circuits, at current ratings from several milliamperes (me) to amperes and higher, it has been found that the closely spaced projecting lead wires offer excellent conditions for the development of high energy arcs.

3,110,787 Patented Nov. 12, 1963 Once a high energy are develops between the lead wires, the lead wires can readily burn clear through the fuse body or, before this happens, the temperature and pressure conditions in the small space within the fuse may rise to a point where the fuse explodes. It is believed that the fuse construction of the invention above described avoids the development of high energy arcs between the lead wires at intermediate power levels for a number of reasons. Firstly, the insulation material extending practically to the end of the lead wires decreases the area of the exposed conductive parts between which the arc may develop, so that the effective are path resistance is increased. Secondly, the presence of a large body of insulation material contiguous to the portions of the fuse filament remaining after the initial disintegration of a portion thereof inhibits the development of a high energy are between the resulting separated portion of the fuse filament. Thirdly, the space between the insulating posts formed by the fuse body insulating material ensures the initial disintegration of the fuse filament at a point spaced furthest from the lead wires and spaced from the insulating material of the fuse body, to minimize oarbonization due to the heating of the fuse filament which would decrease t-he resistance of the potential are discharge path between the lead wires.

In the most preferred form of the invention, to alleviate high energy arcing problems particularly at the higher current levels, an insulating filler material is added above the fuse body so as to embed or encase the fuse filament. Exceptionally good results are obtained with a filler material comp-rising an epoxy resin. it has been discovered that epoxy resins have especially good arc-quenching qualities. Moreover, it has excellent adhesive qualities which are used to aid in holding the fuse cap onto the fuse body under the increased pressure conditions occurring when the filament wire disintegrates under high overload. The problem of cap retention due to the pressures building up in the small volume of the fuse interior is sometimes present even in the absence of high energy arcing. I

fuse shown in FIG. 1;

FIG. 3 is a transverse section through the fuse of FIG. 2, taken substantially along the line 33 therein;

FIG. 4 is an enlarged longitudinal section through a modified miniature fuse forming the most preferred form of the invention;

FIG. 5 is a transverse section through the fuse of FIG. 4, taken substantially along the line 55 therein;

FIGS. 6 and 7 are views illustrating two steps in the fabrication of the fuse of FIGS. 4 and 5 and FIG. 8 is a fragmentary longitudinal section through a still further modified form of the invention.

Reference should first be made to the miniature fuse embodiment of FIGS. 1 through 3. The fuse element there shown is identified generally by reference numeral 2 and includes a fuse body 4 preferably made of a synthetic plastic thermosetting material, such as phenol formaldehyde or similar material. The fuse body preferably has a generally cylindrical shape and is provided with an outwardly extending peripheral flange 5 formed intermediate the ends thereof. A pair of lead wires 66 are molded or otherwise embedded in the fuse body 4-. The lead wires are in spaced juxtaposed relation and extend parallel to the axis of the cylindrical fuse body 4.

In the embodiment of the invention now being dcscribed, the lead Wires 6-4? extend a substantial distance beyond one end of the fuse body 4 thereby to form pigtail connecting leads which may be readily bent in any direction and then soldered to appropriate locations in the circuit in which the fuse is to be connected. (If de sired, however, the lead wires 6-6 may be cut down to a point adjacent the end of the fuse body 4 to form plug connectors which may be insertable into a socket connector, as shown in the embodiment of the invention illustrated in FIGS. 47.) The opposite ends of the lead wires 6-5 are substantially flush with the fuse body 4. The insulating material of the fuse body 4 adjacent the last mentioned ends of the lead wires 6 form short, spaced apart, cylindrical insulating posts 8-8. As illustrated, these posts are spaced a small distance inwardly of the perimeter of the fuse body 4. 1 p

A fuse filament 10 of Wollaston wire or the like and of appropriate size for the current rating desired is welded between the outer faces of the lead wires 6-6. The fuse filament rests on the insulating posts which extend beneath an appreciable extent of the fuse filament.

A cup-shaped member 12 encloses the fuse filament carrying end of the fuse body 4. The cap member may be made of metal or other material. has a cylindrical body portion 14 enclosed at one end by a circular end wall 16. The cylindrical body portion 14 extends around the cylindrical fuse body 4 and the end thereof rests against the annular shoulder 18 formed by the peripheral flange 5. The cap member 1'2 preferably makes a forced fit around the fuse body 4 and is further held securely thereon by staking the same at various points to form tiny nibs 19 biting into the fuse body 4. The cap member is spaced from the fuse filament 10 and thelead wires 6 to which it is connected and defines a space 20 within the cap member.

To illustrate the overall dimensions of an exemplary miniature fuse made in accordance with the present in- 'vention, the diameter thereof was 0.2 inch and the length thereof was 0.270 inch (not including the projecting ends of the lead wires 6). FIGURE 2 is drawn to scale so that other dimensions of the fuse can be obtained therefrom from these overall dimensions.

Reference should now be made to the embodiment of the invention shown in FIGS. 47 which is the preferred form of the invention. This fuse has a fuse body 4 and a cap member 12 which is identical to that previously described in connection with the embodiment of FIGS. 1 through 3. As previously indicated, the fuse now being described has lead wires 66 with short projecting ends on the outside of the fuse body for removably supporting the fuse in a suitable miniature socket connector.

The opposite ends of the lead wires may be flush with the insulating posts 88 of the fuse body or, as illustrated, extend a very short distance therebeyond. The fuse filament 10 of Wollaston wire or the like is welded as before to the outer faces of the fuse filament, and due to the slightly projecting ends of the lead wires, is spaced slightly above the insulating posts.

The major difference between the construction of the fuse now being described and the fuse of FIGS. 1 through 3 is the incorporation of insulating filler material 22 above the fuse body so that it completely envelopes the fuse filament 10. The thickness of the filler material above the fuse filament varies with the current rating of the fuse, the higher the current rating the greater the degree of encasement of the fuse filament. It has been discovered that epoxy resins are exceedingly good arcquenching materials and so is the material used as the filler material. The epoxy resins are especially good adhesive materials also and thus aid materially in securing the cap member 12 on the fuse body so that it will remain thereon under the substantial pressure that builds up in the small space 20 within the fuse when the fuse .is blown.

The cap member The epoxy filler material 22 may be applied to tne fuse body in a number of ways. Most advantageously, however, it is applied in a two step process as illustrated in FIGS. 6 and 7. Before the cap member is applied to prevent the same from running down the side thereof.

The viscous nature of the epoxy material maintains the shape thereof. The uncured epoxy material may be a composition of 14 parts M685 epoxy resin manufactured by the Rubber and Asbestos Corporation of Bloomfield, New Jersey, and one part Cl-I34 curing agent manufactured by the same company. The epoxy material may then be cured at 150 F. for ten minutes. Then, as shown in FIG. 7, a second layer 22 of the same uncured epoxy material is applied over the cured glob 22 of epoxy material, the second layer having an outside diameter somewhat greater than the outside diameter of the fuse body. Then, the cap member 12 is applied around the fuse body where the second layer of epoxy material makes engagement with the inner walls of the cap member. The second layer'of epoxy material may be allowed to cure at rooi temperature for approximately 8 hours. The cap member may be press fitted over the fuse body and staked in the same manner as the cap member of the embodiments of FIGS. 1 through 3. As above indicated, the epoxy material acts as both an arc-quenching and adhesive material which aids in holding the cap member on the fuse body against the high pressures which build up in the fuse.

For lower current ratings where the epoxy material need extend only to the top of the fuse filament 10 as indicated by dotted line 23 in FIG. 2, the epoxy material may be applied in'a single step process. At higher ratings requiring a' much thicker glob of epoxy material, the

viscous uncured epoxy material may run over the sides of the fuse body. In the single step process, the single layer of viscous uncured epoxy material is much thinner and does not run over the sides of the fuse body. In such case, the single layer of epoxy material is applied so that it extendsslightly beyond the outer perimeter of the fuse body so that'it makes engagement with the walls of the cap member when the same is press fitted over the fuse body. The resulting single layer of epoxy material may be cured in this same manner as the second layer of epoxy material described above.

The embodiment of the invention in FIGS. 4 through 7 may have the same overall dimensions as the embodiments of FIGS. 1 through 3. The fuse of the present invention of the size above described, for example, may be designed for use in circuits carrying 5 amperes and higher at volts. The fuse may be designed to'blow, for example, at a minimum overload of percent of rated current in ten seconds or less. The preferred form of the present invention can operate in power circuits capable of providing short circuit currents of 10,000 amperes.

As previously indicated, the fuse filament is made from Wollaston wire which has a platinum core encased in a sheath of silver. For current ratings lower than 60 ma,

the bare platinum wire is quite small and the silver sheath must be etched away leaving a platinum wire of such small diameter (see filament 10" in FIG. 8) that it becomes difiicult to weld the same to the lead wires. In such case, it is more feasible to solder the ends of the fuse filament between extended ends of the lead wires 6-6' as shown in the embodiment of FIG. 8. Low current ratings of this order do not present high energy arcing problems, making unnecessary some of the features of the present invention discussed above including the encasement of the fuse wire l0 in epoxy material or the like. However, to aid in sealing the cap member 12 around the fuse body 4, a layer 22" of epoxy material is applied upon the top of the fuse body 4 below the fuse filament 18 in the same manner as the single layer described above so that the epoxy material seals or secures the cap member 12 around the fuse body. If perchance, a high energy are should develop between the lead wires 66, the arc will be quenched when the lead wires burn down to the level of the epoxy layer 22". The strong adhesive quality of the epoxy material holds the cap member on the fuse body even under the resulting severe high pressure conditions in the small space within the fuse.

It should be understood that numerous modifications may be made of the preferred forms of the invention described above without deviating from the broader aspects of the invention.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A miniature fuse comprising: an integral fuse body made of a solid insulating material in which is embedded a pair of spaced, juxtaposed lead wires extending to opposite ends of said fuse body, one of the corresponding ends of said lead wires forming terminal connections for the fuse, said insulating material of said fuse body forming spaced insulating posts around the opposite end portions of the lead wires, a fuse filament extending between the exposed latter end portions of the lead wires, said fuse filament disintegrating under the heat generated by a given current level, an imperforate cap member enclosing and sealing the fuse filament carrying end of the fuse body, and a solid arc-quenching material filling the space between said fuse body and at least the portions of said fuse filament adjacent to said lead wires, the arc-quenching material occupying only a part of the space in the cap member to leave a pressure-relieving space.

2. A miniature fuse comprising: an integral fuse body made of a solid insulating material through which extends a pair of spaced, juxtaposed lead wires projecting beyond opposite ends of said fuse body, one of the corresponding projecting ends of said lead wires forming exposed terminal connections for the fuse, a fuse filament extending between the other projecting ends of the lead Wires, said fuse filament disintegrating under the heat generated by a given current level, said insulating material of said fuse body extending around the lead wires adjacent the latter projecting ends of the lead wires to form spaced insulating posts, and an imperforate cup-shaped cap member enclosing and sealing the fuse filament carrying end of the fuse body.

3. A miniature fuse comprising: an integral fuse body made of a solid insulating material in which is embedded a pair of spaced, juxtaposed lead Wires extending to opposite ends of said fuse body, one of the corresponding ends of said lead wires forming terminal connections for the fuse, a fuse filament extending between the other ends of the lead wires, said fuse filament disintegrating under the heat generated by a given current level, an imperforate cap member enclosing and sealing the fuse filament carrying end of the fuse body, and a solid arcquenching material bridging the space between said fuse body and said fuse filament around at least the portions of the fuse filament adjacent the ends of said lead wire, the arc-quenching material occupying only a part of the space in the cap member to leave a pressure-relieving space.

4. The fuse of claim 2 wherein the space between the spaced insulating posts is centered beneath the center portion of said fuse filament.

5. The fuse of claim 3 wherein said arc-quenching material is an epoxy resin.

6. The fuse of claim 3 wherein said arc-quenching material extends at least to the center portion of said fuse filament.

References Cited in the file of this patent UNITED STATES PATENTS 1,288,724 Snelling Dec. 24, 1918 2,117,346 Millis May 17, 1938 2,830,156 Burgess Apr. 8, 1958 2,840,668 Yonkers June 24, 1958 2,852,641 Valentine Sept. 16, 1958 2,895,031 Kozacka July 14, 1959 

2. A MINIATURE FUSE COMPRISING: AN INTEGRAL FUSE BODY MADE OF A SOLID INSULATING MATERIAL THROUGH WHICH EXTENDS A PAIR OF SPACED, JUXTAPOSED LEAD WIRES PROJECTING BEYOND OPPOSITE ENDS OF SAID FUSE BODY, ONE OF THE CORRESPONDING PROJECTING ENDS OF SAID LEAD WIRES FORMING EXPOSED TERMINAL CONNECTIONS FOR THE FUSE, A FUSE FILAMENT EXTENDING BETWEEN THE OTHER PROJECTING ENDS OF THE LEAD WIRES, SAID FUSE FILAMENT DISINTEGRATING UNDER THE HEAT GENERATED BY A GIVEN CURRENT LEVEL, SAID INSULATING MATERIAL OF SAID FUSE BODY EXTENDING AROUND THE LEAD WIRES ADJACENT THE LATTER PROJECTING ENDS OF THE LEAD WIRES TO FORM SPLACED INSULATING POSTS, AND AN IMPERFORATE CUP-SHAPED CAP MEMBER ENCLOSING AND SEALING THE FUSE FILAMENT CARRYING END OF THE FUSE BODY. 